The metabolites of arachidonic acid and related fatty acids, collectively termed eicosanoids, exhibit a wide range of biological activities affecting virtually every organ system in mammals. Eicosanoids are among the most important chemical mediators and modulators of the inflammatory reaction and contribute to a number of physiological and pathological processes (See Hardman J. G., Goodman, Gilman A., Limbird L. E.; Goodman & Gilman's The Pharmacological Basis of Therapeutics, 9th edition, McGraw-Hill, N.Y., 1996).
Physiology Pathophysiology and Pharmacological Importance of the Eicosanoids
The eicosanoids are extremely prevalent and have been detected in almost every tissue and body fluid. These lipids contribute to a number of physiological and pathological processes including inflammation, smooth muscle tone, hemostasis, thrombosis, parturition and gastrointestinal secretion. Once synthesized in response to a stimulus, the eicosanoids are not stored to any significant extent but are released immediately and act locally. After they act, they are quickly metabolized by local enzymes to inactive forms. Accordingly, the eicosanoids are categorized as autocrine agents or local hormones. They alter the activities of the cells in which they are synthesized and of adjoining cells. The nature of these effects may vary from one type of cell to another, in contrast with the more uniform actions of global hormones such as insulin, for example. Therefore, the eicosanoids, as local chemical messengers, exert a wide variety of effects in virtually every tissue and organ system.
The principal eicosanoids are the prostaglandins (PG), the thromboxanes (TX) and the leukotrienes (LT), though other derivatives of arachidonate, for example lipoxins, are also produced. They fall into different classes designated by letters and the main classes are further subdivided and designated by numbers.
Inflammatory and Immune Responses
Eicosanoids are lipid mediators of inflammation and play a central, often synergistic, role in numerous aspects of inflammatory responses and host defense. Prostaglandins and leukotrienes are released by a host of mechanical, thermal, chemical, bacterial, and other insults, and they contribute importantly to the genesis of the signs and symptoms of inflammation. The ability to mount an inflammatory response is essential for survival in the face of environmental pathogens and injury, although in some situations and diseases the inflammatory response may be exaggerated and sustained for no apparent beneficial reason. This is the case in numerous chronic inflammatory diseases and allergic inflammation. Acute allergic inflammation is characterized by increased blood flow, extravasation of plasma and recruitment of leukocytes. These events are triggered by locally released inflammatory mediators including eicosanoids and more particularly leukotrienes. The leukotrienes generally have powerful effects on vascular permeability and the leukotriene LTB4 is a potent chemoattractant for leukocytes and promotes exudation of plasma. The prostaglandins PGE2 and PGI2 markedly enhance edema formation and leukocyte infiltration in the inflamed region. Moreover, they potentiate the pain-producing activity of bradykinin.
The participation of arachidonic acid (AA) metabolism in inflammatory diseases such as rheumatoid arthritis, asthma and acute allergy is well established. Prostaglandins have been involved in inflammation, pain and fever. Pathological actions of leukotrienes are best understood in terms of their roles in immediate hypersensitivity and asthma. Lipoxygenases, e.g., 5-lipoxygenase (5-LO), 12-lipoxygenase (12-LO), 15-lipoxygenase A (15-LOA), and 15-lipoxygenase B (15-LOB), have been implicated in the pathogenesis of a variety of inflammatory conditions such as psoriasis and arthritis.
Cardiovascular System
The prostaglandins PGEs, PGF2 and PGD2 cause both vasodilation and vasoconstriction. Responses vary with concentration and vascular bed. Systemic blood pressure generally falls in response PGEs, and blood flow to most organs, including the heart, is increased. These effects are particularly striking in some hypertensive patients. Cardiac output is generally increased by prostaglandins of the E and F series. The importance of these vascular actions is emphasized by the participation of PGI2 and PGE2 in the hypotension associated with septic shock. The prostaglandins also have been implicated in the maintenance of patency of the ductus arteriosus. Thromboxane synthase (TXA2), also known as CYP5, is a potent vasoconstrictor. Leukotriene C4 synthase (LTC4) and the leokotriene LTD4 cause hypotension. The leukotrienes have prominent effects on the microvasculature. LTC4 and LTD4 appear to act on the endothelial lining of postcapillary venules to cause exudation of plasma; they are more potent than histamine in this regard. In higher concentrations, LTC4 and LTD4 constrict arterioles and reduce exudation of plasma.
Blood/Platelets
Prostanoids including prostaglandins and thromboxanes exhibit a wide variety of actions in various cells and tissues to maintain local homeostasis in the body. Eicosanoids modify the function of the formed elements of the blood. PGI2 controls the aggregation of platelets in vivo and contributes to the antithrombogenic properties of the intact vascular wall.
TXA2 is a major product of arachidonate metabolism in platelets and, as a powerful inducer of platelet aggregation and the platelet release reaction, is a physiological mediator of platelet aggregation. Pathways of platelet aggregation that are dependent on the generation of TXA2 are sensitive to the inhibitory action of aspirin, which inhibits the cyclooxygenase (COX) pathway. There has been considerable interest in the elucidation of the role played by prostaglandins and TXA2 in platelet aggregation and thrombosis and by PGI2 in the prevention of these events. The platelet thromboxane pathway is activated markedly in acute coronary artery syndromes and aspirin is beneficial in the secondary prevention of coronary and cerebrovascular diseases. PGI that is generated in the vessel wall may be the physiological antagonist of this system; it inhibits platelet aggregation and contributes to the nonthrombogenic properties of the endothelium. According to this concept, PGI2 and TXA2 represent biologically opposite poles of a mechanism for regulating platelet-vessel wall interaction and the formation of hemostatic plugs and intraarterial thrombi. There is interest in drugs which inhibit thromboxane synthase and modulate PGI2 production.
Smooth Muscle
Prostaglandins contract or relax many smooth muscles beside those of the vasculature. The leukotrienes contract most smooth muscles. In general, PGFs and PGD2 contract and PGEs relax bronchial and tracheal muscle. LTC4 and LTD4 are bronchoconstrictors. They act principally on smooth muscle in peripheral airways and are 1000 times more potent than histamine both in vitro and in vivo. They also stimulate bronchial mucus secretion and cause mucosal edema. A complex mixture of chemical messengers is released when sensitized lung tissue is challenged by the appropriate antigen. Various prostaglandins and leukotrienes are prominent components of this mixture. Response to the leukotrienes probably dominates during allergic constriction of the airway. Evidence for this conclusion is the ineffectiveness of inhibitors of cycloxygenase and of histaminergic antagonists in the treatment of human asthma and the protection afforded by leukotriene antagonists in antigen induced bronchoconstriction. A particularly important role for the cysteinyl-leukotrienes (LTC4, LTD4, and LTE4) has been suggested in pathogenesis of asthma, which is now recognized as a chronic inflammatory condition. They are potent spasmogens causing a contraction of bronchiolar muscle and an increase in mucus secretion.
Gastric and Intestinal Secretions
PGEs and PGI2 inhibit gastric acid secretion stimulated by feeding, histamine or gastrin. Mucus secretion in the stomach and small intestine is increased by PGEs. These effects help to maintain the integrity of the gastric mucosa and are referred to as the cytoprotectant properties of PGEs. Furthermore, PGEs and their analogs inhibit gastric damage caused by a variety of ulcerogenic agents and promote healing of duodenal and gastric ulcers. Cytoprotection is of therapeutic importance and PGE1 analogs are used for the prevention of gastric ulcers.
Kidney and Urine Formation
Prostaglandins modulate renal blood flow and may serve to regulate urine formation by both renovascular and tubular effects. Increased biosynthesis of prostaglandins has been associated with Bartter's syndrome, a rare disease, characterized by urinary wasting of K+.
Leukotrienes have been involved in the pathophysiology of glomerular immune injury.
Reproduction and Parturition
Much interest is attached to the possible involvement of prostaglandins in reproductive physiology. Lowered concentrations of prostaglandins in semen have been implicated in male infertility. Prostaglandins are also thought to contribute to the symptoms of primary dysmenorrhea. Inhibitors of cyclooxygenase are effective in relieving the symptoms of this condition. Elevated levels of prostaglandins are involved in onset of labor. Inhibitors of cyclooxygenase increase the length of gestation and interrupt premature labor.
Cancer Metastasis
Tumors in animals and certain spontaneous human tumors are accompanied by increased concentrations of local or circulating prostaglandins. Eicosanoids have been shown to be involved in various aspects of neoplasia including cell transformation, tumor promotion, tumor cell growth, and metastasis. Some studies have implicated platelet aggregation and the effects of prostaglandins and hydroxyeicosatetraenoic acid (12-HETE) in the hematogenous metastasis of tumors.
Many of the products of arachidonic acid metabolism are potent mediators of physiological responses and contribute to disorders of development, cellular function, tissue repair, and host defenses in a number of diseases.
Arachidonic Acid Metabolism and Biosynthesis of Eicosanoids
The primary source of eicosanoids in mammalian systems is the metabolic products of arachidonic acid. After stimulation by trauma, infection, or inflammation, translocated phospholipases, especially phospholipase A2, act on membrane phospholipids to liberate arachidonic acid. Once released, arachidonate is metabolized to oxygenated products by several distinct enzyme pathways, including cyclooxygenases, several lipoxygenases, and cytochrome P450s (CYP). The specific enzyme pathway involved determines, which products are formed.
Release of Arachidonic Acid from Cell Membranes and its Regulation
The eicosanoids are a family of substances produced from the polyunsaturated fatty acid arachidonic acid, which is present in plasma-membrane phospholipids. The first rate-limiting step in the biosynthesis of eicosanoids is the release of arachidonic acid from the membrane, a process that is mainly catalyzed by cytosolic phosholipase A2 (cPLA2). The synthesis of eicosanoids begins when a stimulus such as a hormone, a neurotransmitter, a drug or a toxic agent activates cytosolic phospholipase A2. This arachidonic acid specific phospholipase plays a major role in the cell signaling events that initiate the arachidonate cascade. One important trigger of arachidonate release and eicosanoid synthesis involves tissue injury and inflammation.
The activities of many enzymes are regulated by calmodulins (CAL) that serve as calcium sensors in eukaryotic cells. The binding of Ca2+ to multiple sites in calmodulin induces a major conformational change that converts it from an inactive to an active form. Activated calmodulin then binds to many enzymes and target proteins in the cell, modifying their activities and thereby regulating various metabolic pathways. Calmodulins are involved in a number of processes regulated by Ca2+ including smooth muscle contraction, neurotransmission, apoptosis, cell cycle progression and gene expression. Calmodulins also participate in the regulation of arachidonate release. They directly stimulate cytosolic phospholipase A2, whereas calmodulin antagonists inhibit enzyme activity and the release of arachidonic acid.
Annexins (ANX) are a family of multifunctional calcium and phospholipid-binding proteins, they belong to a family of proteins that interact with phospholipids in a Ca2+ dependant manner. Annexins have been implicated in the pathogenesis of benign and malignant neoplasms of different origins. Moreover, several annexins have also been involved in autoimmune diseases such as systemic lupus erythematosus, rheumatoid arthritis and inflammatory bowl disease. Numerous physiological functions have been attributed to annexins including regulation of membrane traffic during exocytosis and endocytosis, mediation of cytoskeletal-membrane interactions, membrane receptor function, regulation of membrane-dependent enzymes, mitogenic signal transduction, transmembrane ion channel activity, cell-cell adhesion, antiinflammatory properties, inhibition of blood coagulation and inhibition of phospholipase A2. Annexins have been suggested as regulators of prostaglandin metabolism and of the arachidonate cascade as a result of their inhibitory effect on phospholipase A2. It is still a matter of debate as to whether inhibition of phospholipase A2 is the result of calcium-dependent sequestration of phospholipids (substrate depletion mechanism) or a direct effect of the annexins acting via protein-protein interactions. Calpactin I (light chain) is the cellular ligand of annexin II and induces its dimerization. Annexin II and calpactin I (CALPA) constitute a calcium binding complex composed of two light chains (calpactin I) and two heavy chains (annexin II). Calpactin I may function as regulator of annexin II phosphorylation.
The activities of phospholipase A2, annexins and calmodulins are common points of regulation in the formation of all eicosanoids.
Downstream of phospholipase A2, the varying eicosanoid-pathway enzymes found in particular cell types determine which eicosanoids are synthesized in response to particular stimuli.
Cyclooxygenase Pathway
This pathway initiated by cyclooxygenase (COX) leads ultimately to formation of the cyclic endoperoxides, prostaglandins (PG), and thromboxanes (TX).
There are two isoforms of the cyclooxygenase, COX-1 and COX-2. The former is constitutively expressed in most cells. In contrast, COX-2 is not normally present but may be induced by certain factors such as cytokines and growth factors. The cyclooxygenases have two distinct activities: an endoperoxidase synthase activity that oxygenates and cyclizes the unesterified precursor fatty acid to form the cyclic endoperoxide PGG and a peroxidase activity that converts PGG to PGH. PGG and PGH are chemically unstable, but they can be transformed enzymatically into a variety of products, including PGI, TXA2, PGE, PGF or PGD. Isomerases lead to the synthesis of PGE2 and PGD2, whereas PGI2 is formed from PGH2 through prostacyclin synthase. TXA2 is formed by thromboxane synthase. Although most tissues are able to synthesize the PGG and PGH intermediates from free arachidonate, the fate of these precursors varies in each tissue and depends on the complement of enzymes that are present and on their relative abundance. For example, lung and spleen are able to synthesize the whole range of products. In contrast, platelets contain thromboxane synthase as the principal enzyme that metabolizes PGH, while endothelial cells contain primarily prostacyclin synthase.Lipoxygenase Pathways
Lipoxygenases are a family of cytosolic enzymes that catalyze the oxygenation of fatty acids to corresponding lipid hydroperoxides. Arachidonate is metabolized to HPETE (hydroperoxyeicosatetraenoic acid), which is then converted either enzymatically or non-enzymatically to 12-HETE (hydroxyeicosatetraenoic acid). HPETEs may further be converted to hepoxilins and lipoxins. Lipoxygenases differ in their specificity for placing the hydroperoxy group, and tissues differ in the lipoxygenases they contain. These enzymes are referred to as 12-, 15-, 5- and 8-lipoxygenases according to the oxygenation sites in arachidonic acid as substrate.
The lipoxygenases catalyze reactions and generate products of potential relevance to membrane remodeling, cell differentiation and inflammation. Products of the 15-LO pathway could contribute to the pathophysiology of allergic airway inflammation while products of the 12-LO pathway have been implicated in cancer metastasis, psoriasis and inflammation.
Various biological activities have been reported for the 12-lipoxygenase metabolites of arachidonic acid. As other eicosanoids, they are important chemical mediators and modulators of the inflammatory reaction. 12-HETE is the major arachidonic acid metabolite of 12-lipoxygenase and seems to be implicated in a wide-spectrum of biological activities such as stimulation of insulin secretion by pancreatic tissue, suppression of renin production, chemoattraction of leukocytes and initiation of growth-related signaling events, such as activation of oncogenes, protein kinase C, and mitogen-activated protein kinases. 12-lipoxygenase activity and 12-HETE production are also important determining factors in tumor cell metastasis and have been implicated in human prostate cancer and breast cancer (Honn et al., Cancer Metastasis Rev., 13:365-396, 1994, Gao et al., Adv. Exp. Med. Biol., 407:41-53, 1997; Natarajan et al., J. Clin. Endocr. Metab., 82:1790-1789, 1997,). Further, 12-HETE has also been implicated in inflammatory skin diseases such as psoriasis (Hussai et al., Am. J. Physiol., 266:243-253, 1994). As mentioned above, metabolism of arachidonic acid by 12-lipoxygenase further generates lipoxins and hepoxillins. Lipoxins play the role of both immunologic and hemodynamic regulators and a variety of biological activities have been reported for hepoxillins which are related to the release of intracellular calcium and the opening of potassium channels (Yamamoto et al., Pro. Lipid Res., 36:23-41, 1997).
The 5-lipoxygenase (5-LO) is perhaps the most important of these enzymes since it leads to the synthesis of leukotrienes. Activation of the 5-LO enzyme involves its docking to a protein termed 5-lipoxygenase-activating protein (FLAP). This binding activates the enzyme, results in its association with the cell membrane and increased synthesis of 5-HPETE and leukotrienes. Leukotriene A (LTA) synthase is associated with 5-lipoxygenase and promotes the rearrangement of 5-HPETE to an unstable intermediate LTA4; which may be transformed to LTB4 by leukotriene A4 hydrolase (LTA4H); alternatively, it may be conjugated with glutathione by LTC4 synthase to form LTC4. LTA4 hydrolase is a pivotal element in leukotriene biosynthesis. Omega-oxidation is regarded as the major pathway for the catabolism of LTB4. This reaction is catalyzed by LTB4 omega-hydroxylase (LTB4H3) also called CYP4F2. LTD4 is produced by the removal of glutamic acid from LTC4 and LTE4 results from the subsequent cleavage of glycine; the reincorporation of glutamic acid yields LTF4.
Epoxygenase Pathway
Arachidonate is metabolized to a variety of metabolites by enzymes that contain cytochrome P450. The epoxygenase pathway of the arachidonic acid cascade leads to the formation of epoxyeicosatrienoic acids (EETs) and dihydroxyeicosatrienoic acids (DHETs). CYP2J2 is a human cytochrome P450 arachidonic acid epoxygenase expressed in extrahepatic tissues and particularly in the intestine. In addition to the known effects on intestinal vascular tone, CYP2J2 products may be involved in the release of intestinal neuropeptides, control of intestinal motility and modulation of intestinal fluid/electrolyte transport.
Eicosanoid Receptors
The diversity of the effects of eicosanoids is explained by the existence of a number of distinct receptors that mediate their actions. All prostaglandin receptors identified to date are coupled to effector mechanisms through G proteins. Distinct receptors for leukotrienes also have been identified in different tissues, all of these appear to activate phospholipase C.
Therapeutic Agents Interacting with Arachidonic Acid Metabolism
Because of their involvement in so many disease states, there has been a considerable effort to develop effective inhibitors to the formation or action of the eicosanoids. The drugs that influence the eicosanoid pathways are the most commonly used drugs in the world today. Their major uses are to reduce pain, fever and inflammation. Several classes of drugs, most notably the nonsteroidal antiinflammatory drugs (NSAIDs) owe their therapeutic effects to blockade of the formation of eicosanoids. Selective inhibitors of arachidonic acid metabolism also have an important therapeutic value. Inhibition of cyclooxygenase (COX), the enzyme responsible for the biosynthesis of the prostaglandins and certain related autacoids, generally is thought to be a major facet of the mechanism of NSAIDs. Aspirin and newer, widely used drugs belong to the NSAIDs. All NSAIDs are antipyretic, analgesic and antiinflammatory but there are important differences in their activities and in their side effects. The reasons for such differences are not fully understood. Side effects of these drugs include gastrointestinal ulceration, disturbances in platelet function, changes in renal function and hypersensitivity reactions. It is now appreciated that there are two forms of cyclooxygenase (COX), inhibition of COX-2 is thought to mediate the antipyretic, analgesic and antiinflammatory action of NSAIDs, whereas the simultaneous inhibition of COX-1 may result in unwanted side effects. Efforts are under way to identify COX-2 specific agents. But, it is also possible that enhanced generation of lipoxygenase products, due to the diversion of arachidonic acid metabolism from the cyclooxygenase pathway towards the lipoxygenase pathways, contributes to some of the side effects. Effort is being devoted to a search for drugs that will produce more selective interventions by acting farther along the biosynthetic pathways. Several compounds have been described that selectively antagonize responses to TXA2 and to PGH2. Some are receptor antagonists others directly inhibit thromboxane synthase.
Advances in understanding the pathobiology of the inflammatory process has suggested several novel approaches for development of drugs to block this process. These include phospholipase A2 inhibitors. Glucocorticoids are thought to have an effect on arachidonic acid metabolism through the induction of lipocortin that inhibits phospholipase A2.
NSAIDs generally do not inhibit the formation of other eicosanoids such as the lipoxygenase-produced leukotrienes. Substantial evidence indicates that leukotrienes contribute to the inflammatory response through a variety of effects. Leukotrienes have been implicated as mediators of inflammation and immediate hypersensitivity reactions—in particular, human bronchial asthma—and thus considerable effort has been done to develop either inhibitors of the production or blockers of the action of the actions of these mediators. Various therapeutic approaches have been used including 5-lipoxygenase inhibitors, which block leukotriene formation, or cysteinyl leukotriene receptor antagonists, which block receptor function. LTC4 synthase is another key step in biosynthesis of leukotrienes and represents another possible site for therapeutic intervention. Drugs targeting leukotriene biosynthesis are being tested and used for their utility in the treatment of various inflammatory conditions.
Most of these drugs are efficacious in providing relief but all available agents have associated, and sometimes severe, toxicity. Certain individuals display intolerance to aspirin and to other drugs acting on arachidonic acid metabolism; this is manifest by symptoms that range from liver toxicity, gastric and intestinal ulceration, disturbance in platelet function, renal injury, nephritis, vasomotor rhinitis with profuse watery secretions, angioneurotic edema, generalized urticaria, and bronchial asthma to laryngeal edema and bronchoconstriction, hypotension, and shock. The underlying mechanism for these severe side effects is not known. Moreover, while these agents have been highly useful for treatment of acute, self-limited inflammatory conditions; their ability to modify disease progression in chronic inflammatory settings remains an area of controversy. The complexity of the highly regulated pathways and enzymes that lead to the formation of the eicosanoids, has limited the precise identification of the metabolites and enzymes in the arachidonic acid cascade, which play the causal role in pathologies or in side effects to some drugs.
Pharmacogenomics and Arachidonic Acid Metabolism
The vast majority of common diseases, such as cancer, hypertension, diabetes and some inflammatory diseases are polygenic, meaning that they are caused by multiple genes. In addition, these diseases are modulated by environmental factors such as pollutants, chemicals and diet. This is why many diseases are called multifactorial; they result from a synergistic combination of factors, both genetic and environmental. Therapeutic management and drug development could be markedly improved by the identification of specific genetic polymorphisms that determine and predict patient susceptibility to diseases or patient responses to drugs.
To assess the origins of individual variations in disease susceptibility or drug response, pharmacogenomics uses the genomic technologies to identify polymorphisms within genes which are part of biological pathways involved in disease susceptibility, etiology, and development, or more specifically in drug response pathways responsible for a drug's efficacy, tolerance or toxicity. It can provide tools to refine the design of drug development by decreasing the incidence of adverse events in drug tolerance studies, by better defining patient subpopulations of responders and non-responders in efficacy studies and, by combining the results obtained therefrom, to further allow better enlightened individualized drug usage based on efficacy/tolerance prognosis. Pharmacogenomics can also provide tools to identify new targets for designing drugs and to optimize the use of already existing drugs, in order to either increase their response rate and/or exclude non-responders from corresponding treatment, or decrease their undesirable side effects and/or exclude from corresponding treatment patients with marked susceptibility to undesirable side effects. However, for pharmacogenomics to become clinically useful on a large scale, molecular tools and diagnostics tests must become available.
Inflammatory reactions, which are involved in numerous diseases, are highly relevant to pharmacogenomics both because they are at the core of many widespread serious diseases, and because targeting inflammation pathways to design new efficient drugs includes numerous risks of potentiating serious side effects. Arachidonic acid metabolism is particularly relevant since its products, the eicosanoids, are powerful inflammatory molecules and play a role in a number of physiological functions.
Genetic Analysis of Complex Traits
Until recently, the identification of genes linked with detectable traits has relied mainly on a statistical approach called linkage analysis. Linkage analysis is based upon establishing a correlation between the transmission of genetic markers and that of a specific trait throughout generations within a family. Linkage analysis involves the study of families with multiple affected individuals and is useful in the detection of inherited-traits, which are caused by a single gene, or possibly a very small number of genes. Linkage analysis has been successfully applied to map simple genetic traits that show clear Mendelian inheritance patterns and which have a high penetrance (the probability that a person with a given genotype will exhibit a trait). About 100 pathological trait-causing genes have been discovered using linkage analysis over the last 10 years.
But, linkage studies have proven difficult when applied to complex genetic traits. Most traits of medical relevance do not follow simple Mendelian monogenic inheritance. However, complex diseases often aggregate in families, which suggests that there is a genetic component to be found. Such complex traits are often due to the combined action of multiple genes as well as environmental factors. Such complex trait, include susceptibilities to heart disease, hypertension, diabetes, cancer and inflammatory diseases. Drug efficacy, response and tolerance/toxicity can also be considered as multifactoral traits involving a genetic component in the same way as complex diseases. Linkage analysis cannot be applied to the study of such traits for which no large informative families are available. Moreover, because of their low penetrance, such complex traits do not segregate in a clear-cut Mendelian manner as they are passed from one generation to the next. Attempts to map such diseases have been plagued by inconclusive results, demonstrating the need for more sophisticated genetic tools.
Knowledge of genetic variation in the arachidonic acid cascade is important for understanding why some people are more susceptible to disease involving arachidonic acid metabolites or respond differently to treatments targeting arachidonic acid metabolism. Ways to identify genetic polymorphism and to analyze how they impact and predict disease susceptibility and response to treatment are needed.
Although the genes involved in arachidonic acid metabolism represent major drug targets and are of high relevance to pharmaceutical research, we still have scant knowledge concerning the extent and nature of sequence variation in these genes and their regulatory elements. For example, the cDNA and part of the genomic sequence for human 12-lipoxygenase have been cloned and sequenced (Izumi et al., Proc. Natl. Acad. Sci. USA, 87:7477-7481, 1990; Funk et al., Proc. Natl. Acad. Sci. USA, 87:5638-5642, 1990; Yoshimoto et al., Biochem. Biophys. Res. Commun., 172:1230-1235, 1990, Yoshimoto, et al., J. Biol. Chem., 267:24805-24809, 1992). However, the complete genomic sequence of the 12-lipoxygenase, including its regulatory elements, have not been described.
In the cases where polymorphisms have been identified, the relevance of the variation is rarely understood. While polymorphisms hold promise for use as genetic markers in determining which genes contribute to multigenic or quantitative traits, suitable markers and suitable methods for exploiting those markers have not been found and brought to bare on the genes related to arachidonic acid metabolism.